Doppler-Based Renal Resistive Index Can Assess Progression of Acute Kidney Injury in Patients Undergoing Cardiac Surgery Pierre-Gre ´ goire Guinot, MD,* Euge ´ nie Bernard,* Osama Abou Arab,* Louise Badoux,* Momar Diouf,† Elie Zogheib, MD,* and Herve ´ Dupont, PhD, MD* Objectives: The objective of this study was to test whether assessment of renal resistive index measured after cardiac surgery (RRI T0 ) can diagnose persistent acute kidney injury (AKI). The predictive value was evaluated using a gray-zone approach. Design: A prospective observational study. Setting: A teaching university hospital. Participants: Eighty-two patients following cardiac sur- gery with cardiopulmonary bypass. Interventions: Measurements of hemodynamic parameters and RRI were obtained before surgery, on admission to the intensive care unit, 6 hours after admission, and on the first postoperative day. AKI was defined according to the renal risk, injury, failure, loss of kidney function, end-stage of kidney disease (RIFLE) classification during the first postoperative week. Persistent AKI was defined as AKI lasting 43 days. Measurements and Main Results: Out of the 82 patients, 15 (18%) developed persistent AKI, and 6 (7%) developed transient AKI. The median value and time-course of RRI were significantly different between patients with transient AKI and persistent AKI. Doppler-based RRI T0 predicted persistent AKI with an area under the receiver-operating characteristic curve of 0.93 (95% confidence interval: 0.85- 0.98, p o 0.0001). The optimal cut-off of RRI was 0.73 (95% confidence interval: 0.73-0.75). The gray-zone approach identified a range of RRI values between 0.72 and 0.75 in 14% of patients. Conclusions: Doppler-based RRI can be helpful for non- invasive assessment of renal function recovery after cardiac surgery by using RRI T0 to predict persistent AKI. The optimal cut-off was 0.73 with a gray zone ranging between 0.72 and 0.75. & 2013 Elsevier Inc. All rights reserved. KEY WORDS: acute kidney injury, cardiac surgical procedures, Doppler, postoperative period, ultrasonography A CUTE KIDNEY INJURY (AKI), which occurs com- monly in as many as 30% of patients after cardiac surgery, is associated with increased morbidity and mortality. AKI is the consequence of an interplay of pathophysiologic mechanisms, including several factors. Preoperative adminis- tration of radiocontrast agent, cardiopulmonary bypass (CPB), ischemia-reperfusion, hemodynamic impairment, systemic inflammatory response, and red blood cell transfusion can damage the renal parenchyma. 1-3 Postoperative AKI is char- acterized by a progressively worsening course comprising several phases. The early phase is characterized by alterations in vasoreactivity and renal perfusion. 3 In clinical practice, the diagnosis of AKI is based on serum and urinary markers, such as serum creatinine (sCr), urinary output, and fractional excretion of sodium or urea. 4,5 These markers are insensitive, unreliable, and can be altered by several factors during the postoperative period. 6 Several authors have studied novel biomarkers for earlier detection of AKI, such as cystatine C and neutrophil gelatinase-associated lipocalin. However, bed- side use of these biomarkers may be limited by their cost and predictability. 7-9 The Doppler-based renal resistive index (RRI; [peak systolic flow velocity  minimum diastolic flow velocity]/[peak systolic flow velocity]) measured by transpar- ietal renal Doppler can assess renal perfusion. Over the past decade, some authors have emphasized the role of RRI as an earlier predictor of AKI in a wide range of clinical situa- tions. 10-16 Early elevation of RRI has been associated with the development of acute tubular necrosis and allograft survival. 11 Platt et al suggested that RRI might distinguish acute tubular necrosis from prerenal failure. 12 Later, Darmon et al demon- strated that RRI might be helpful in predicting the reversi- bility of AKI in a mixed critical care population. Recently, Bossard et al demonstrated a good predictive value of RRI measured immediately after surgery to predict delayed AKI. 16 However, these authors selected a population with risk factors for AKI, and their approach did not differentiate AKI according to its duration and etiology. 16 In the area of cardiac surgery, AKI frequently presents a continuum between volume-responsive AKI and nonvolume-responsive AKI rather than 2 distinct entities. Because the duration of AKI has been associated with improved long-term survival after cardiac surgery, 17 early characterization of renal perfusion alterations associated with AKI and its duration may allow physicians to apply an appropriate treatment that could imp- rove recovery from AKI. The objective of this study was to assess the value of RRI measured at admission to the intensive care unit (ICU) to distinguish transient from persistent AKI. This indicator was also evaluated using a gray-zone approach. From the Departments of *Anaesthesiology and Critical Care Medicine, yBiostatistics, and Clinical Research, Amiens University Hospital, Place Victor Pauchet, Amiens, France. The authors performed this study in the course of their normal duties as full-time employees of public healthcare institutions. The study was performed at Amiens University Hospital. This study has been approved by the IRB of Amiens University Hospital (Comite ´ de Protection des Personnes Nord Ouest, Amiens, France). Address reprint requests to Pierre-Gre ´goire Guinot, Pole d’Anesthe ´sie-Re ´animation, CHU d’Amiens, Place Victor Pauchet, 80054 Amiens cedex, France. E-mail: guinotpierregregoire@ gmail.com & 2013 Elsevier Inc. All rights reserved. 1053-0770/2601-0001$36.00/0 http://dx.doi.org/10.1053/j.jvca.2012.11.024 890 Journal of Cardiothoracic and Vascular Anesthesia, Vol 27, No 5 (October), 2013: pp 890–896